The present invention relates to belt tensioners, and more particularly to a belt tensioner for use in automotive vehicles for automatically tensioning an endless belt of the drive system for such vehicle accessories as a water pump, a power steering pump, an alternator and the like.
In such a belt tensioner as described above, an initial tensioning force on the endless belt is adjusted by displaced position of an idler pulley or the vehicle accessories in engagement with the belt so that the belt does not slip in operation of the vehicle accessories under the maximum loaded condition. In operation of the vehicle accessories, however, a tensioning force on the belt for reliable power transmission to the vehicle accessories will differ in accordance with the driving condition of the vehicle, i.e. a relatively small tensioning force on the belt is required to transmit a small drive torque to the vehicle accessories from a prime mover of the vehicles, while a large tensioning force on the belt is required to transmit a large drive torque to the vehicle accessories from the prime mover. If a large drive torque is transmitted to the vehicle accessories under an insufficient initial tensioning force on the belt, there will occur slippage of the belt. If an excessive initial tensioning force acts on the belt, power transmission efficiency of the belt will decrease to increase fuel consumption.
As a result of experiments, it has been observed that a relationship between power transmission efficiency of the endless belt and total drive torque for the vehicle accessories in consideration with tensioning force on the belt is obtained as shown by characteristic curves A, B and C in FIG. 1. From the characteristic curves, it will be understood that when the total drive torque for the vehicle accessories is relatively small, a small tensioning force on the belt is effective to increase the power transmission efficiency of the belt, as is shown by the characteristic curve A. When the total drive torque for the vehicle accessories increases, medium and large tensioning forces on the belt are effective in sequence to increase the power transmission efficiency of the belt, as is shown by the characteristic curves B and C in FIG. 1. Based on the above facts, it is consequently noted that the maximum efficiency of power transmission in relation to the total drive torque for the vehicle accessories is obtainable by adjustment of the tensioning force on the belt as defined by a characteristic curve D in FIG. 1.